1. Technical Field
The invention relates to vehicle axles and wheel end assemblies, and in particular to axles and wheel end assemblies for heavy-duty vehicles, such as tractor-trailers. More particularly, the invention is directed to a heavy-duty axle spindle and wheel end assembly that includes a precision-formed axle, wheel hub, and axle spindle nut which cooperate to enable the axle spindle nut to achieve a precise position and thus provide a desirable light preload on the bearings of the wheel end assembly.
2. Background Art
For many years, the heavy-duty vehicle industry has utilized wheel end assemblies which are mounted on each end of one or more non-drive axles. Each wheel end assembly typically includes a hub rotatably mounted on a bearing assembly that in turn is immovably mounted on the outboard end of the axle, commonly known as an axle spindle. The bearing assembly includes an inboard bearing and an outboard bearing, which may be separated by a bearing spacer. An axle spindle nut assembly secures the bearing assembly on the axle spindle by threadably engaging threads that are cut into the outer diameter of the outboard end of the axle spindle. In addition to retaining the position of the bearings and any spacer, the axle spindle nut assembly may be used to provide a clamp force to compress the hearings, and any bearing spacer, to a predetermined amount.
As is well known to those skilled in the art, for normal operation of the wheel end assembly to occur, the bearing assembly and surrounding components must be lubricated with grease or oil. Therefore, the wheel end assembly also must be sealed to prevent leakage of the lubricant, and also to prevent contaminants from entering the assembly, both of which could be detrimental to its performance. More specifically, a hubcap is mounted on an outboard end of the wheel hub adjacent to and outboard from the axle spindle nut assembly, and a main seal is rotatably mounted on an inboard end of the hub and the bearing assembly in abutment with the axle spindle, resulting in a closed or sealed wheel end assembly.
While most wheel end assemblies include these general features, the design and arrangement of the hub, bearing assembly, bearing spacer, axle spindle nut assembly, hubcap, main seal, and other components, as well as the axle spindle, vary according to the specific vehicle design and its anticipated uses. Moreover, the design and construction of prior art axle spindles, wheel hubs and axle spindle nuts exhibit certain disadvantages associated with installation and maintenance of an optimum position of the spindle nut to provide the proper clamp force to compress the bearings.
More particularly, the clamp force that compresses the bearings involves the placement of force by the axle spindle nut on the cones of the bearings and any spacer between the bearings, which are known in the art as a bearing cone and spacer group. For the purpose of convenience, reference herein shall be made to the bearing cone and spacer group with the understanding that such reference includes applications which utilize a bearing spacer, and applications which do not utilize a bearing spacer. Placement of the proper amount of force on the bearing cone and spacer group helps to optimize the life of the bearings by controlling the tolerance range of the end play of the hearings.
For example, if the position of the axle spindle nut does not create a sufficient clamp force on the bearing cone and spacer group, there may be excessive end play of the bearings, which in turn creates excessive axial end play of the wheel end assembly relative to the axle spindle. Such excessive end play may allow undesirable movement of the main seal, which in turn potentially reduces the life of the main seal and the bearings. If the position of the axle spindle nut creates a clamp force on the bearing cone and spacer group that is too high, the bearings may effectively be over-compressed, interfering with their rotation and causing them to potentially wear out prematurely.
An optimum position of the axle spindle nut creates an optimum clamp force on the bearing cone and spacer group that ideally places the bearing cone and spacer group into what is known in the art as light preload. Light preload is an optimized compression of the bearing cone and spacer group that effectively is between a lack of clamp force that results in some axial end play of the wheel end assembly, and a higher clamp force which over-compresses the bearings. Light preload is advantageous when compared to a lack of clamp force that results in even a small amount of axial end play, because it restricts axial movement of the wheel end assembly, and thus significantly enhances and extends the fatigue life of the bearings and the main seal. Light preload is also advantageous when compared to a clamp force that results in over-compression of the bearings, since the light preload does not over-compress the bearings, and thus extends their fatigue life. The extension of bearing and main seal life desirably reduces the cost, effort and time that are required to remove a vehicle from service to replace worn bearings and main seals.
However, placing a light preload on the bearing cone and spacer group involves a very narrow and precise range of clamp force, which requires an extremely precise position of the axle spindle nut, making the light preload condition extremely difficult to achieve and maintain. In the prior art, the development of components that can consistently and reliably create a uniform light preload condition has not been accomplished. This has been due primarily to the lack of a precision-formed axle, wheel hub, and axle spindle nut which cooperate to enable the spindle nut to consistently achieve and maintain a precise position, and thus a light preload condition. As a result, manufacturers have instead designed axle spindle and wheel end assemblies to employ a position of the axle spindle nut that results in a certain amount of axial end play, having determined that, while not optimal, it is a more desirable condition than a position of the axle spindle nut which results in a clamp force that over-compresses the hearings.
For example, on prior art axle spindle and wheel end assemblies that employ wheel hubs with high-end, specialized unitized bearings or bearing cartridge systems, undesirably high tightening torques in excess of 500 foot-pounds are required. Such high torques do not allow consistent repeatability of a precise axle spindle nut position and thus a proper clamp force that is able to maintain a light preload condition. As a result, such axle spindle and wheel end assemblies must instead employ a range of adjustment or position of the axle spindle nut which includes axial end play at one end of the range, up to a clamp force that creates a light preload condition at the other end of the range. For example, the range may be from about 0.001 inches of axial end play to about 0.001 inches of preload on the most precise systems, and from about 0.003 inches of axial end play to about 0.003 inches of preload on less precise systems.
Other axle spindle and wheel end assemblies employ more standard bearings with a bearing spacer. These assemblies have a moderate tightening torque, but the tolerances associated with the manufacturing of the bearing spacer preclude the repeatable precision that is needed to achieve a light preload condition. As a result, such axle spindle and wheel end assemblies employ a range of adjustment or position of the axle spindle nut which includes axial end play of the wheel end assembly from about 0.001 inches to about 0.006 inches. Such a range of adjustment or position of the axle spindle nut is preferable to attempting a light preload with an imprecise system, thereby ensuring that the system does not create an unknown preload condition that over-compresses the bearings.
Still other axle spindle and wheel end assemblies have even less repeatable precision of the position of the axle spindle nut. Due to such imprecision, these assemblies also employ a position of the axle spindle nut that results in axial end play of the wheel end assembly, so as to avoid the inadvertent creation of an unknown and non-measurable preload condition that may over-compress the bearings.
Even when a relatively precise axle spindle nut is employed, such as a spindle nut having features as described in U.S. Patent Application Publication No. 2009/0245969, which is owned by the same assignee as the present application, Hendrickson USA, L.L.C., a repeatable light preload condition cannot be consistently obtained unless the axle spindle and wheel hub interfaces are precisely and accurately formed to cooperate with one another and with the spindle nut.
More particularly, a wheel hub must be formed with a bearing surface for the inboard bearing and a bearing surface for the outboard bearing. In the prior art, many manufacturers of wheel hubs have machined or finished the bore for one bearing surface from one direction using a first fixture to hold the hub, and have then machined or finished the bore for the second bearing surface from another direction by turning the wheel hub over and using a second fixture to hold the huh. This process of using two separate fixtures is referred to in the art as a multi-chuck system, and such multi-chuck systems use separate locating surfaces to machine the bore for the first bearing surface and the bore for the second bearing surface. In a multi-chuck system, a machining chip or other contaminant could be captured on one of the locating surfaces, which in turn causes the bores for the bearing surfaces to be machined out of alignment.
If this misalignment is not severe, the wheel hub may still function acceptably when the axle spindle and wheel end assembly is set to operate with axial end play, as excessive compression of the bearings due to the misalignment may not be experienced. However, the optimum bearing fatigue life is not obtainable using such a wheel hub in conjunction with an axle spindle nut position that allows axial end play. In addition, if a clamp force that achieves a light preload is attempted with such a hub, it is likely that the bearings will inadvertently be overloaded or over-compressed due to non-uniform loading caused by the aforementioned misalignment of the bores of the bearing surfaces.
In addition, the prior art forming processes for axle spindles contribute to the lack of precision of the system. That is, the axle spindle includes a bearing surface for the inboard bearing, a bearing surface for the outboard bearing, and threads for the axle spindle nut. In many cases, the spindle threads are not cut at the same time as the bearing surfaces are finished. For example, some axle manufactures cut the spindle thread into the axle spindle before friction welding the spindle to an axle central tube to save processing time, and will then finish the bearing surfaces after the friction welding process. Friction welding the axle spindle to the axle central tube may create variation in alignment of the opposite spindles on the axle, which results in the spindle threads being tipped or out of alignment relative to the bearing surfaces on the spindle. When the wheel end assembly is set to operate with axial end play relative to the axle, an axle with spindle threads that are not aligned with the bearing surfaces on the spindle may still function acceptably. However, if a clamp force that achieves a light preload is attempted, it is likely that the bearings will inadvertently be overloaded or over-compressed due to non-uniform loading caused by the misalignment of the axle spindle threads to the spindle bearing surfaces.
Moreover, even a relatively precise axle spindle nut must have certain features in order to ensure that the system is precise enough to obtain a consistent light preload condition. More particularly, axle spindle nuts typically do not include threads that are accurately or precisely positioned perpendicular to the inboard surface of the nut, due to separate machining processes and/or separate locating surfaces for machining the inboard surface of the nut and for forming the threads in the nut. Since axle spindle nuts of the prior art typically do not include threads that are precisely perpendicular to the inboard surface of the nut, consistent achievement of a light preload condition is precluded.
As a result, prior art axle spindle and wheel end assemblies lack the necessary precision on the critical surfaces of the axle spindle, wheel hub and axle spindle nut to enable the axle spindle nut to achieve a precise position and thus in turn achieve a consistent, desirable light preload condition in clamping the bearing cone and spacer group of the wheel end assembly. These disadvantages of prior art axle spindle and wheel end assemblies make it desirable to develop a heavy-duty axle spindle and wheel end assembly that includes a precision-formed axle, wheel hub, and axle spindle nut which cooperate to enable the axle spindle nut to consistently achieve a precise position and thus provide a light preload on the bearing cone and spacer group of the wheel end assembly. The present invention satisfies these needs, as will be described below.